1. Field of the Invention
The present invention relates to a column electrode drive circuit of a liquid crystal display device, and more particularly to a column electrode drive circuit of an active matrix type liquid crystal display device.
2. Description of the Prior Art
Conventionally, an active matrix type liquid crystal display device in which a switching transistor is additionally provided for each pixel of a display panel has been extensively used as a display device for a pocketable information apparatus or the like for the reason that a high contrast can be obtained by the switching function of the transistor even when multi-line multiplex drive is executed.
FIG. 14 is a block diagram showing the generic construction of the active matrix type liquid crystal display device.
Referring to FIG. 14, a switching transistor 1-d is connected to a pixel 1-c provided at an intersection of a row electrode 1-a and a column electrode 1-b on one of two substrates which are arranged opposite to each other at a specified distance to constitute part of an LCD panel 1. A row electrode drive circuit 2 sequentially applies to each row electrodes 1-a a scanning pulse for sequentially turning on each switching transistor 1-d connected to each of plural number of row electrodes 1-a arranged in parallel with each other. Meanwhile, a column electrode drive circuit 3 applies simultaneously to every column electrode 1-b a voltage corresponding to the density of an image to be displayed on each pixel 1-c relevant to one row electrode 1-a.
A control circuit 4 controls the row electrode drive circuit 2 and the column electrode drive circuit 3 to display an image on a pixel matrix on the LCD panel 1.
FIG. 11 is a block diagram showing the internal construction of the column electrode drive circuit 3 shown in FIG. 14, while FIGS. 12a-12f are timing charts of signals in the column electrode drive circuit 3.
The following describes the operation of the column electrode drive circuit 3 based on FIGS. 11 and 12a to 12f.
Referring to FIG. 11, the column electrode drive circuit 3 is composed of a shift register circuit 5, a sampling circuit 6, a hold circuit 7, and a level selector circuit 8. The shift register circuit 5, sampling circuit 6, hold circuit 7, and level selector circuit 8 are each composed of a number of elements corresponding to the number of "n" of the column electrodes 1-b.
A display signal D input to each element of the sampling circuit 6 is a signal representing the density of an image to be displayed on each pixel. The display signal D is normally input serially one pixel by one pixel in a manner as shown in FIG. 12c. Therefore, by means of the shift register circuit 5 and the sampling circuit 6, display signals D1 through Dn in an interval corresponding to the pixels 1-c on which the image is to be displayed are extracted from the display signal D input serially.
The shift register circuit 5 sequentially shifts a sampled signal SP in accordance with a shift clock CK to yield output signals SP1 through SPn from each element thereof to the corresponding element of the sampling circuit 6. Then each element of the sampling circuit 6 sequentially samples the display signals D1 through Dn corresponding to respective column electrodes 1-b according to the output signals SP1 through SPn input thereto, and outputs sampled display signals S1 through Sn to the corresponding element of the hold circuit 7.
Each element of the hold circuit 7 simultaneously outputs the display signals S1 through Sn as display signals H1 through Hn to the corresponding element of the level selector circuit 8 in synchronization with a hold signal LS. Simultaneously with the above-mentioned operation, the hold circuit 7 holds the display signals H1 through Hn.
Each element of the level selector circuit 8 selects a voltage at a level to be supplied to each column electrode 1-b among "m" number of levels of input voltages V1 through Vm supplied externally based on the input display signals H1 through Hn, and outputs the voltage at the selected level as each of voltage signals Y1 through Yn to the corresponding column electrode 1-b.
When the voltages V1 through Vm based on the voltage signals Y1 through Yn are thus applied to the column electrodes 1-b, the voltages V1 through Vm are applied to the pixels 1-c via each switching transistor 1-d which is in "ON" state as connected with the row electrode 1-a to which the scanning pulse is applied by the operation of the row electrode drive circuit 2. As a result, a variable-density image corresponding to the display signals D1 through Dn is displayed on one line of the pixel matrix of the LCD panel 1.
FIG. 13 is a generic circuit diagram of the hold circuit 7.
The hold circuit 7 has D-flip-flops 9, 10, . . . , 11 as the aforementioned elements. Then, in synchronization with the hold signal LS input to clock terminals CK of the D-flip-flops 9 through 11, the sampled display signals S1 through Sn which are input to data terminals D are simultaneously output as the display signals H1 through Hn, while the display signals H1 through Hn are held.
As described above, the column electrode drive circuit 3 of the liquid crystal display device simultaneously applies the voltages V1 through Vm which have the levels selected by the level selector circuit 8 in synchronization with the timing of the hold signal LS to the liquid crystals of each pixel via the switching transistor 1-d which is in "ON" state according to the scanning pulse from the row electrode drive circuit 2.
In other words, the voltages are simultaneously applied to the column electrodes 1-b in units of scanning of the row electrode 1-a.
There is a display-integrated type tablet device obtained by incorporating a tablet function in addition to the aforementioned image display function into the aforementioned liquid crystal display device.
In the display-integrated type tablet device, one field is divided into an image display period in which an image is displayed on the pixel matrix of the LCD panel and a coordinate detection period in which the coordinate values at a designated point on the LCD panel are detected. In the coordinate detection period, a scanning pulse is sequentially applied to the row electrodes 1-a and the column electrodes 1-b, and the scanning pulse applied to each electrode is detected by means of an electronic pen which is electrostatically coupled with each row electrode 1-a and each column electrode 1-b.
Then, according to the scanning timing of each electrode group and a time to the detection of the scanning pulse by the electronic pen, the coordinate values at the tip end of the electronic pen on the LCD panel are detected.
In the above case, the detection of the x-coordinate at the tip end of the electronic pen is executed by sequentially scanning the column electrodes 1-b in the coordinate detection period. Therefore, it is indispensable for the column electrode drive circuit of the display-integrated type tablet device to have a function of simultaneously applying display voltages to the column electrodes in units of scanning of a row electrode in the image display period as well as a function of scanning the column electrodes by sequentially applying the scanning voltage to the column electrodes in the coordinate detection period.
Unfortunately, as described above, the column electrode drive circuit 3 of the conventional liquid crystal display device has only the function of simultaneously changing in synchronization with the hold signal LS the voltages of the voltage signals Y1 through Yn output to all the column electrodes, and therefore the voltages of the voltage signals Y1 through Yn cannot be changed sequentially.
For the above reasons, there has been the problem that the row electrode drive circuit 2 of the conventional liquid crystal display device cannot be utilized as a column electrode drive circuit for the display-integrated type tablet device.
Even when the scanning pulse for the x-coordinate detection is sequentially applied to the column electrodes, since the shift clock signal CK has an excessively high shift speed in the conventional liquid crystal display device when the device is not modified, and a time difference in the change of electric potential between an arbitrary column electrode and the adjacent column electrode is about several hundred nanoseconds, it is very difficult to detect the x-coordinate value of an arbitrary column electrode based on a relation in phase between the change of electric potential at the column electrode and a horizontal sync signal, and the like.